Abstract/Summary

Southern Kermadec arc stratovolcanoes (of predominantly basaltic and andesitic composition) provide a depth transect of the transition between effusive and explosive submarine volcanism. Observations along 4.6 km of towed-camera track from the crests and upper flanks of the Clark and Rumble III volcanoes reveal a consistent pattern of substrate types that are interpreted to record effusive and explosive eruption processes. Below 700 m water depth, substrates are dominated by massive/blocky flows, pillow lavas, pillow and angular block talus, localized sheet flows, and minor granule-sand volcaniclastic detritus. The latter typically forms a substrate mode of 5–20%. Above 450 m, sand lapilli (that is in part winnowed and rippled), have a substrate mode of 50–100%, with minor components of massive flows, pillow lavas, and coarse talus. This difference in abundance of sand lapilli between 450 and 700 m is interpreted to record the transition between effusive and explosive (both phreatic and phreatomagmatic) eruptions. Between 600 and 700 m, a varied distribution of sand-lapilli abundance is interpreted as recording a mix of effusive and explosive eruptions, consistent with possible differences in the physical interaction of magma–water and known compositional magma heterogeneity.